Mesothelin has been suggested as a therapeutic target because it is highly expressed in malignant mesotheliomas (Chang et al., Cancer Res 52:181-186, 1992; Chang and Pastan, Proc Natl Acad Sci USA 93:136-140, 1996) and other solid tumors, such as stomach cancer, squamous cell carcinomas, prostate cancer, pancreatic cancer, lung cancer, cholangiocarcinoma, breast cancer and ovarian cancer (Hassan et al., Clin. Cancer Res. 10:3937-3942, 2004; McGuire et al., N. Engl. J. Med. 334:1-6, 1996; Argani et al., Clin. Cancer Res. 7:3862-3868, 2001; Hassan et al., Appl. Immunohistochem. Mol. Morphol. 13:243-247, 2005; Li et al., Mol. Cancer Ther. 7:286-296, 2008; Yu et al., J Cancer 1:141-1749, 2010; Tchou et al., Breast Cancer Res Treat 133(2):799-804, 2012; U.S. Pat. No. 7,081,518).
The mesothelin (MSLN) gene encodes a ˜70 kDa precursor protein that is processed to a ˜30 kDa N-terminal protein and a ˜40 kDa C-terminal membrane-bound mature mesothelin (Hassan and Ho, Eur J Cancer 44:46-53, 2008). Over the last two decades, a number of anti-mesothelin monoclonal antibodies (mAbs) have been developed, including SS1P immunotoxin and MORAb-009 (also known as amatuximab), which are currently being evaluated in clinical trials for mesothelioma and other cancers (Hassan and Ho, Eur J Cancer 44:46-53, 2008; Ho, Biodrugs 25:275-284, 2011). SS1P is a recombinant immunotoxin consisting of a murine anti-mesothelin Fv fused to a truncated Pseudomonas exotoxin that mediates cell killing (Pastan and Hassan, Nat Rev Cancer 6:559-565, 2006). A clinical trial of SS1P in combination with chemotherapy is currently ongoing. MORAb-009, a chimeric (mouse/human) antibody based on the murine SS1 Fv, elicits antibody-dependent cell-mediated cytotoxicity (ADCC) on mesothelin-bearing tumor cells (Hassan et al., Cancer Immun 7:20, 2007).
Investigators at the U.S. National Cancer Institute (NCI) recently generated two fully human mAbs (m912 and HN1) that recognize mesothelin (Feng et al., Mol Cancer Ther 8:1113-1118, 2009; Ho et al., Int J Cancer 128:2020-2030, 2011). The HN1 human Fv was isolated from a phage display library and a fully human IgG was generated. An HN1 immunotoxin was also generated by fusing the HN1 Fv to a truncated Pseudomonas exotoxin A (PE38) (Ho et al., Int J Cancer 128:2020-2030, 2011). HN1 IgG binds to cell surface-associated mesothelin and kills cancer cells with very strong ADCC. The HN1 human antibody recognizes a conformational epitope overlapping the SS1 site in mesothelin, indicating that HN1 can be developed as a fully human version of SS1-based mAbs (such as MORAb-009). Despite the number of known mesothelin mAbs available, none have shown complement-dependent cytotoxicity (CDC) against tumor cells. Therefore, current mesothelin-targeted therapy is hampered by the lack of anti-mesothelin mAbs with potent CDC.
CDC is an important mechanism of cell killing for therapeutic antibodies (Weiner et al., Nat Rev Immunol 10:317-327, 2010). The first approved mAb for cancer therapy, rituximab, is partially dependent on CDC for its anti-tumor activity. In preclinical studies, its antitumor activity was completely abolished in C1q-deficient mice (Di Gaetano et al., J Immunol 171:1581-1587, 2003). Depletion of complement also decreased its activity in a xenograft model of B cell lymphoma (Cragg and Glennie, Blood 103:2738-2743, 2004). It has been suggested that CDC may occur when the antibody binding site is close to the cell membrane (Pawluczkowycz et al., J Immunol 183:749-758, 2009). As evidence of this, ofatumumab, which binds much closer to the cell membrane than rituximab, also has much higher CDC activity (Pawluczkowycz et al., J Immunol 183:749-758, 2009). Almost all of the existing mesothelin mAbs and immunotoxins (including HN1 and SS1P/MORAb-009) recognize Region I, the N-terminal end of cell-surface mesothelin presumed to be located far from the cell membrane (Kaneko et al., J Biol Chem 284:3739-3749, 2009).